U.S. Pat. No. 4,028,236 discloses a method of removing mercury from caustic effluent streams using tin-coated iron strips on which metallic mercury accumulates and is removed. Low mercury levels in the solution were reported for quite long treatment times.
DE - A - 3'721'141 proposes to remove mercury from flue-scrubber wash water using tin chloride as reducing agent to bring the mercury level to about 0.1 mg/l.
DE - A - 2,610,826 discloses a wet scrubber system using a tin or zinc grid to remove mercury from off gases.
DE - A - 2,501,375 describes the recovery of mercury by passing an aqueous sludge through tin or zinc pellets in a hollow body.
All these non-electrolytic methods have the drawback that they rely on an exchange reaction that leads to substantial tin dissolution with consequent contamination of the outlet stream, especially at low pH. Also, low mercury contamination levels cannot be reached especially when organic or low solubility inorganic compounds are treated. Further, the methods are mainly for caustic solutions in which the materials precipitate, but such precipitation leads to blockage of the filters used and the filtered product is itself a non disposable contaminant.
DE - A - 3'709'359 discloses the electrolytic recovery of metallic mercury from a bath containing Hg.sub.2 Cl.sub.2 in suspension by cathodic reduction using a cathode of iron, silver, nickel, copper, cadmium, aluminium, zinc, tin or their alloys in which the steady-state conditions are such that metallic mercury collects on and drips off the cathode. The presence of metallic mercury on the cathode surface means that non-negligeable amounts of colloidal mercury necessarily remain dissolved in the solution, which prevents removal of mercury below a limiting value.
UK - A - 1 312 681 describes the recovery of mercury and other heavy metals using a porous cathode, usually of graphite, with current reversal to deposit the heavy metal from a dilute stream and re dissolve it in a more concentrated solution, the metal being then recovered form this concentrated solution.
EP - B - 0'071'443 describes an electrolytic cell using reticulate cathodes made of metallic foams and open structure coated titanium anodes wherein a plurality of spaced anodes and cathodes are arranged as a membrane-free and diaphragm-free cell through which the waste waters flow. The metal removing efficiency of the cell is good and the final concentrations of heavy metals contained in the waste waters can be reduced from a few grams per liter to a few ppm or less.
Other designs of electrolytic cells for metal recovery from waste streams are also known, including cells with packed or fluidized bed electrodes and cells with a roll configuration.
Generally speaking, it can be said that for many highly contaminated solutions (containing much more than ten ppm of mercury), using known electrolytic methods and cells the mercury level can be reduced to just a few ppm without great difficulty.
However, in practice, the known electrolytic cells have not proven to be effective in applications involving the removal of mercury to trace values, e.g. to below about 1 ppm (1000 ppb). For this, methods involving the use of ion-exchange resins are used despite the fact that such resins and their regeneration are expensive. These resins are not effective against organic mercury-laden effluents but nevertheless have been used to reduce the mercury content of such streams from 1-2 ppm to about 200 ppb.